This invention pertains to the field of drive systems for causing a conduit of a vibrating-tube-process-parameter sensor to oscillate. In particular, the present invention pertains to a system operating in the modal domain to drive any number of drivers with any modal force pattern.
It is known to use Coriolis effect mass flowmeters to measure mass flow and other information for materials flowing through a conduit. Exemplary Coriolis flowmeters are disclosed in U.S. Pat. Nos. 4,109,524 of Aug. 29, 1978, U.S. Pat. No. 4,491,025 of Jan. 1, 1985, and Re. 31,450 of Feb. 11, 1982, all to J. E. Smith et al. These flowmeters have one or more conduits of straight or curved configuration. Each conduit configuration in a Coriolis mass flowmeter has a set of natural vibration modes, which may be of a simple bending, torsional, radial or coupled type. Each conduit is driven to oscillate at resonance in one of these natural modes. Material flows into the flowmeter from a connected conduit on the inlet side of the flowmeter, is directed through the conduit or conduits, and exits the flowmeter through the outlet side. The natural vibration modes of the vibrating, material filled system are defined in part by the combined mass and stiffness characteristics of the conduits and the material flowing within the conduits.
When there is no flow through the flowmeter, all points along the conduit oscillate, due to an applied driver force, with identical phase or a zero-flow phase depending on the mode of the driven vibration. As material begins to flow, Coriolis forces cause a change in phase difference between any two points along the conduit. The phase on the inlet side of the conduit lags the driver, while the phase on the outlet side leads the driver. Pick-off sensors are placed on the conduit to produce sinusoidal signals representative of the motion of the conduit. Signals output from the pick-off sensors are processed to determine the change in phase difference between the pick-off sensors. The change in phase difference between two pick-off sensor signals is proportional to the mass flow rate of material through the conduit.
A typical component of every Coriolis flowmeter, and of every vibrating tube densitometer, is the drive or excitation system. The drive system operates to apply a periodic physical force to the conduit which causes the conduit to oscillate. The drive system includes at least one driver mounted to the conduit(s) of the flowmeter. The driver mechanism typically contains one of many well known arrangements, such as, but not limited to, a voice coil where a magnet is mounted to one conduit and a wire coil is mounted to the other conduit in an opposing relationship to the magnet. A drive circuit continuously applies a periodic, typically sinusoidally or square shaped, drive signal to the driver coil. Through interaction of the continuous alternating magnetic field produced by the coil in response to the periodic drive signal and the constant magnetic field produced by the magnet, both flow conduits are initially forced to vibrate in an opposing sinusoidal pattern which is thereafter maintained. Those skilled in the art recognize that any device capable of converting an electrical signal to mechanical force is suitable for application as a driver. (See U.S. Pat. No. 4,777,833 issued to Carpenter and assigned on its face to Micro Motion, Inc.) Also, one need not use a sinusoidal signal but rather any periodic signal may be appropriate as the driver signal (see U.S. Pat. No. 5,009,109 issued to Kalotay et. al. and assigned on its face to Micro Motion, Inc.).
A typical mode, although not the only mode, in which Coriolis flowmeters are driven to vibrate is the first out-of-phase bending mode. The first out-of-phase bending mode is the fundamental bending mode at which the two tubes of a dual tube Coriolis flowmeter vibrate in opposition to one another. However, this is not the only mode of vibration present in the vibrating structure of a Coriolis flowmeter driven in the first out-of-phase bending mode. There are, of course, higher modes of vibration which may be excited. There is also, as a result of fluid flowing through the vibrating flow tube and the consequent Coriolis forces, a first out-of-phase twist mode that is excited as well as other modes. There are also in-phase and lateral modes of vibration. Ultimately, there are hundreds of vibration modes actually excited in a Coriolis flowmeter that is driven to oscillate in the first out-of-phase bending mode. Even within relatively narrow range of frequencies near the first out-of-phase bending mode there are at least several additional modes of vibration. In addition to multiple modes being excited by the driven excitation of the flow tubes, modes can be excited due to vibrations external to the flowmeter. For example, a pump located elsewhere in a process line might generate a vibration along a pipeline that excites a mode of vibration in a Coriolis flowmeter. Another reason that additional and undesirable modes are sometimes excited in a Coriolis flowmeter is when manufacturing tolerances are such that the driver elements are not located symmetrically on the flow tubes. This results in the driver putting eccentric forces into the flow tubes hence exciting multiple modes of vibration. Thus a Coriolis flowmeter driven to oscillate or resonate at the first out-of-phase bending mode actually has a conduit(s) oscillating in many other modes in addition to the first out-of-phase bending mode. Meters driven to oscillate in a different mode than the first out-of-phase bending mode experience the same phenomenon of multiple excited modes in addition to the intended drive mode.
Existing drive systems process a feedback signal, typically one of the pick-off sensor signals, to produce the drive signal. Unfortunately, the drive feedback signal contains responses from other modes in addition to the desired mode of excitation. Thus, the drive feedback signal is filtered through a frequency domain filter to remove unwanted components and the filtered signal is then amplified and applied to the driver. However, the frequency domain filter used to filter the drive feedback signal is not effective at isolating the single desired drive mode from other mode responses present in the drive feedback signal. There can be off-resonance responses from other modes which are near the desired mode resonance frequency. There might also be resonant responses at frequencies approaching the desired resonance frequency. In any event, the filtered drive feedback signal, i.e., the drive signal, typically contains modal content at frequencies other than just the desired mode for excitation of the flow tube. A drive signal composed of resonant response from multiple modes inputs, through the driver, energy to the flow tube that excites each mode for which the drive signal contains modal content. Such a multi-mode drive signal causes operational problems in Coriolis flowmeters. Further, frequency domain filters introduce phase lag in the filtered drive signal. This can result in a requirement for higher drive power to drive the flow tube at the desired amplitude.
One problem caused by a multi-mode drive signal is that external vibrations such as pipeline vibrations are reinforced by the drive signal. If pipeline vibrations external to the Coriolis flowmeter cause the flowmeter to vibrate, the drive feedback signal contains the response to the pipeline vibration. The frequency domain filter fails to remove the undesired response if the pipeline vibration falls at least in part within the frequency pass band of the filter. The filtered drive feedback signal, including the undesired response to the pipeline vibration, is amplified and applied to the driver. The driver then operates to reinforce the excitation mode of the pipeline vibration.
An additional problem of a drive signal having modal content at multiple frequencies occurs with respect to the density measurement made by a Coriolis mass flowmeter. The density measurement in a Coriolis flowmeter or vibrating tube densitometer relies on the measurement of the resonant frequency of the vibrating flow tube. A problem arises when the flow tube is driven in response to a drive signal containing modal content at multiple modes. The superposition of the multiple modes in the drive signal can result in a flow tube that is driven off-resonance from the true resonant frequency of the desired drive mode. An error in the density measurement can result.
The above-noted problems describe various conditions under which a drive signal intended to excite a single mode is degraded so that it excites multiple modes. Co-pending application Ser. No. 08/890,785 (the ""785 application) filed Jul. 11, 1997, by applicant Timothy J. Cunningham (now U.S. Pat. No. 6,199,022), teaches the use of a modal filter to produce a drive signal from at least two feedback signals where the modally-filtered drive signal has modal content at only the desired mode of vibration. A modal filter is used to enhance a desired drive mode and suppress one or more undesired modes. The ""785 application teaches, for the first time, a drive system that produces a drive signal having modal content only at the desired drive mode.
There are situations where one wants to excite multiple modes and thus a drive signal having modal content at multiple modes is required. See for example, U.S. Pat. No. 5,734,112 to assignee Micro Motion, Inc., where two modes are excited and changes in the ratio of the resonant frequencies of the two excited modes are related to fluid pressure within the vibrating conduit. Simultaneous excitation of two modes requires a drive signal having modal content at those two modes. There may be other reasons for driving a vibrating conduit such that multiple modes are excited and multiple modes are suppressed. U.S. Pat. No. 6,092,429 teaches a superposition in the temporal domain of one drive signal having a modal content at a first mode and a second drive signal having modal content at a second mode to produce a dual-mode drive signal. U.S. Pat. No. 6,092,429 also teaches the optimum location of drivers on a vibrating flowtube such that undesired modes are not excited and particularly teaches the advantages of multiple drivers. There are no existing drive control systems for producing drive signals having desired modal content at multiple modes or for producing modally-filtered drive signals to multiple drivers.
There exists a need for a drive control system that is easily adaptable to process a plurality of drive feedback signals to produce a plurality of drive signals each of which has modal content that influences multiple modes.
The above identified problems, and others, are solved and a technical advance achieved in the field by the generalized modal space drive control system of the present invention. The present invention provides a generalized drive control system that operates in the modal domain to produce a drive signal or signals for one or more drivers on a vibrating structure. The drive system receives multiple drive feedback signals, decomposes the modal content of the vibrating conduit into Single-Degree-Of-Freedom (SDOF) modal response signals, processes the SDOF modal response signals to select the desired amounts of each mode, and transforms the results to the physical domain for application to the driver(s). Thus, using the generalized drive control system of the present invention, one or more drive signals are produced to excite one or more drivers thereby exciting or suppressing certain modes on the vibrating structure. In addition, the drive control system of the present invention operates in, and easily switches between, multiple operating configurations. For example, one set of drive signals is generated by the present invention during a flow measurement operating configuration and an alternative set of drive signals is generated by the present invention during an axial stress measurement operating configuration.
The drive control system of the present invention is used to control the vibration modes of a vibrating structure that includes at least one vibrating conduit. The vibrating structure might also include additional vibrating conduits or one or more balance bars that are not wetted by the fluid to be measured by the flowmeter. Also, the vibrating structure might include flanges or cases, for example. Feedback sensors and drivers are placed on any portion(s) of the vibrating structure, according to the present invention, to bring about the excitation of desired modes and the suppression of undesired modes.
In one embodiment of the present invention, at least one feedback sensor provides a motion signal indicative of the movement of the vibrating conduit. The motion signal has modal content at multiple modes each of which is associated with a certain frequency. Multiple frequency bandpass filters produce multiple modal response signals. Each modal response signal is associated with one of the modes of vibration present on the vibrating conduit. In another embodiment of the present invention, at least two feedback sensors provide motion signals indicative of the movement of the vibrating conduit. The motion signals have modal content at a plurality of vibration modes. The motion signals are input to a multi-channel modal filter having a channel for a every mode to be influenced, i.e., excited or suppressed, by the drive control system. Each channel of the modal filter produces a modal response signal corresponding to one of the plurality of vibration modes at which the conduit vibrates. A drive controller having a channel for each modal response signal operates to produce a modal excitation signal corresponding to each vibration mode. The modal excitation signal corresponding to a given vibration mode indicates the extent to which the given mode is present in the final drive signal(s). A modal-to-physical force projector receives the modal excitation signals, transforms them to the physical domain and outputs one or more drive signals for application to one or more drivers. A further embodiment of the present invention utilizes frequency bandpass or other temporal filters in addition to modal filters in order generate modal response signals for further processing. In a further embodiment of the present invention, a combination of frequency band-pass filters and modal filters are used. The single output from each modal filter channel is passed through a frequency domain filter.
The modal filter of the present invention is comprised of a channel for each vibration mode influenced by the drive system. A modal response signal that is a substantially SDOF signal is produced for each vibration that is influenced by the drive system of the present invention. The configuration of the modal filter channels, once set, need not be reconfigured even when the drive system switches from a first operating configuration to a second operating configuration. Likewise, the modal-to-physical force projector is configured according to various methods disclosed herein and need not be reconfigured even when the drive system switches from a first operating configuration to a second operating configuration.
Each channel of the drive controller receives an input of one of the SDOF modal response signals. The modal response signal is compared to a modal response setpoint to produce a mode error signal. The mode error signal is amplified by a gain stage to produce a modal excitation signal for each mode influenced by the drive system of the present invention. The relative amplitude of the modal excitation signals corresponding to each influenced vibration mode indicates the extent to which a given mode contributes to the drive signal(s) produced by the present drive system. For example, if a given vibration mode, an xe2x80x9cundesiredxe2x80x9d mode, is to be suppressed, then the modal response setpoint for the corresponding modal response signal is zero. The resulting modal excitation signal is transformed to the physical domain, i.e., an applied force, by operation of the modal-to-physical force projector. The force is applied to the vibrating tube to suppress the undesired mode.
The drive system of the present invention provides for multiple operating configurations. Thus, a Coriolis flowmeter, for example, utilizing the drive system of the present invention can alternate between various operating configurations each of which requires a different drive methodology. Consider the following example. In a first operating configuration, a certain vibration mode may be undesirable and therefore suppressed by the drive system of the present invention. However, in a second operating configuration, the same vibration mode may be desirable and therefore enhanced by the drive system of the present invention. The only changes to the drive system of the present invention from one operating configuration to another operating configuration are the modal response setpoint for each channel of the drive controller. A look-up table contains the appropriate setpoints for the various operating configurations of a given Coriolis flowmeter or vibrating densimeter.
The drive system of the present invention transforms multiple motion signals from the temporal domain to the modal domain. In the modal domain, the motion signals are decomposed into a SDOF modal response signal for each vibration mode to be influenced by the drive system. Each modal response signal is processed to produce a corresponding modal excitation signal. The modal excitation signal represents the magnitude of the corresponding vibration mode that is applied as a component of the drive signal(s) to bring about the desired modal response setpoint. The modal excitation signals are transformed from the modal domain to the temporal domain to produce a drive signal(s) that results in a force applied to a driver(s).